For industrial PSA tape applications it is very common to employ double-sided PSA tapes in order to join two materials to one another. A distinction is made here, depending on type, between single-layer double-sided self-adhesive tapes and multilayer double-sided self-adhesive tapes.
Single-layer double-sided self-adhesive tapes, known as transfer tapes, are constructed such that the PSA layer contains no carrier and is lined only with corresponding release materials, such as siliconized release papers or release films. Transfer tapes may be lined with release materials on one side or both sides. Often use is made of release papers or release films with different degrees of siliconization on either side, so that the transfer tape can be wound readily into a roll and then also applied readily. Adhesive transfer tapes are frequently used in order to provide any of a very wide variety of substrates with pressure-sensitive adhesion. This is accomplished, for example, by laminating the transfer tape onto the substrate. In that case the release paper remains as a liner to the PSA layer in the product.
Relatively thin transfer tapes are often produced with self-adhesive compositions from solution, whereas thicker transfer tapes are produced with self-adhesive compositions from the melt or by means of what is called UV polymerization. In this procedure a prepolymerized syrup composed of acrylate monomers is coated between two UV-transparent, antiadhesively coated release films and is crosslinked on the web by UV irradiation. Specifications that may be mentioned by way of example include U.S. Pat. No. 4,181,752, EP 084 220 A, EP 202 938 A, EP 277 426 A, and U.S. Pat. No. 4,330,590. A disadvantage of this technology is the often high residual monomer fraction in the self-adhesive compositions. This fraction of residual monomer is unacceptable for many applications. Transfer tapes filled with non-UV-transparent adjuvants cannot be produced in this way.
DE 43 03 183 A1 also describes a method of producing thick PSA layers, especially for producing high-performance self-adhesive articles. In said process a mixture of starting monomers which is to be polymerized by means of UV radiation is mixed, and thickened in the process, with a solvent-free, saturated photopolymerizable polymer, and then this mixture is applied to a dehesively treated carrier and exposed to UV radiation. A disadvantage is the use of copolymerized or added photoinitiators, since the layers may undergo yellowing and, in the event of UV exposure prior to use, an often marked change in the technical adhesive properties is found. In that case it is necessary to go to considerable effort and expense—by means, for example, of UV-impervious packaging—to ensure that the customer obtains a uniformly high bonding performance. Moreover, in the event of bonding on UV-transparent substrates, such as on window glass or transparent plastic surfaces, for example, there is a risk that layers containing photoinitiator will undergo aftercrosslinking. This does result initially in an increase in bond strength, but further crosslinking causes the layers to become paintlike and undergo embrittlement. Sooner or later, this leads to the failure of the bond, particularly under a shearing load.
Transfer tapes may be foamed or filled in order to improve their properties, particularly for example in respect of bonding to uneven substrates. DE 40 29 896 A1 describes a carrierless, double-sided self-adhesive tape comprising a pressure-sensitive adhesive layer more than 200 μm thick which contains solid glass microballs of more than 1.5 g/cm3 in density. This tape is said to exhibit particularly effective adhesion. A disadvantage is the high density as a result of the glass balls that are used.
Double-sided adhesive tapes of multilayer construction have advantages over their single-layer counterparts, since the variation of the individual layers allows specific properties to be set. For instance, a three-layer adhesive tape, consisting of a middle carrier layer and two outer layers, can be constructed symmetrically or asymmetrically. The two outer layers may each be PSA layers, or, for example, one layer may be a PSA layer and the other layer a heat-activatable adhesive. The carrier, i.e., the middle layer, may for example be a film, a nonwoven, a “non-woven” material or a foam film carrier. Foam or foam like carriers are often used when there is a requirement for high bond strength to uneven surfaces or when distances are to be compensated.
For instance, for adhesive assembly tapes, use is often made of closed-celled foam carriers based on PE (polyethylene), PU (polyurethane) or EVA (ethyl-vinyl acetate), which have a double-faced coating of synthetic rubber PSA or acrylate PSA. Applications, listed by way of example, are the bonding of mirrors, trim strips and emblems in automotive construction, further uses in automobile construction, and also use in the furniture industry or in household appliances.
Assembly tapes for the exterior sector generally possess PSAs based on polyacrylate. This material is particularly weathering-resistant and very long-lived, and is virtually inert toward UV light and toward degradation by oxidation or ozonolysis.
Also known are adhesive assembly tapes with middle layers of rubber, styrene block copolymers, and polyurethane. All of these materials fail to possess the same good aging and thermal stability properties of polyacrylate. Systems based on acrylate block copolymers are resistant to aging but are not sufficiently heat-resistant for high-performance requirements, since these systems are crosslinked only physically by way of styrene or methyl methacrylate domains. When the softening temperature of the domains is reached (as in the case of styrene block copolymers), the PSAs soften. Consequently, the bond fails.
Another disadvantage of typical foam adhesive tapes is that they can easily split. If, for example, PE foam is used, this material softens on heating to about 100° C., and the bond fails. Double-sided assembly tapes of this kind are unsuitable for high-grade applications. Foams based on PU are indeed more temperature-stable, but have a tendency to yellow under UV and sunlight exposure. They too are often unsuitable for high-performance applications.
For a number of years there have been double-sided adhesive tapes available which are of three-layer construction with an acrylate core. This viscoelastic acrylate core is foamlike. Its foamlike structure is achieved through the admixture of hollow glass or polymer balls to the acrylate composition, or else the acrylate composition is foamed by means of expandable polymeric “microballoons”. Provided adjacent to this viscoelastic layer are in each case PSAs, based in the majority of cases likewise on acrylate, rarely on synthetic rubber, or else in special cases on heat-activatable adhesive layers. The advantages of the viscoelastic acrylate core arise on the one hand from the physical properties of the polyacrylate (which, as already mentioned, are a particular weathering stability and long life, and substantially inert behavior toward UV light and toward degradation by oxidation or ozonolysis). As a result of the design of the acrylate core layer, determined for example by the comonomer composition, nature and proportion of certain fillers, and the degree of crosslinking, these products are especially suitable for bonding articles to substrates having uneven surfaces. Depending on the choice of PSA, a broad spectrum of properties and bond strengths can be covered.
Nevertheless, as a result of their preparation, the aforementioned systems have critical disadvantages. The viscoelastic acrylate core layer is prepared by a process of two-stage UV polymerization. In the first step of that process a mixture based on acrylate monomers, 10% by weight acrylic acid and 90% by weight isooctyl acrylate for example, is prepolymerized to a conversion of approximately 10%-20% by UV irradiation in a reactor in the presence of a photoinitiator. Alternatively, this “acrylic syrup” can also be obtained by thermally initiated free radical polymerization. In the second step this acrylic syrup, often after further photoinitiator, fillers, hollow glass balls, and crosslinker have been added, is coated between antiadhesively coated UV-transparent films, and is polymerized to a higher degree of conversion on the web, by means of repeated UV irradiation, and in the course of this polymerization it is crosslinked. The completed three-layer product is obtained, for example, after the PSA layers have been laminated on.
The production of “relatively thick” viscoelastic layers in particular must in many cases be carried out in the absence of oxygen. In that case the composition is protected by a lining of film material, and UV initiation takes place through the films. PE and PP films which are sometimes used for this purpose deform under the conditions of crosslinking reaction (in the case of UV-initiated polymerization, heat of reaction is liberated, and can cause deformation of non-temperature-resistant film) and are therefore poorly suited. UV-transparent films such as PET are more thermally stable; in this case, however, it is necessary to add to the composition a photoinitiator which reacts to longwave radiation, in order for the reaction to take place. As a consequence of this, these layers have a tendency to undergo aftercrosslinking under UV light or sunlight. This process negates the advantage specific to the polyacrylate as a material. A further disadvantage is that fillers not transparent to UV cannot be used. Moreover, as a result of the process, there remains a high residual monomer fraction in these products. Possible reduction of residual monomer through a reduction in coating speed or through intensive subsequent drying is not very economic. The maximum achievable layer thickness is very heavily dependent on the wavelength of the photoinitiator used. Layers can be produced of up to about 1 mm, albeit with the disadvantages specified above. Layers any thicker than this are virtually impossible to obtain.
A particular disadvantage in the case of acrylate layers produced by two-stage UV polymerization, UV crosslinking or electron beam treatment is a more or less strongly pronounced profile of crosslinking through the layer. Toward the irradiation source, the UV-crosslinked layer is always more strongly crosslinked than on the side opposite the UV radiation source. The degree of the crosslinking profile is dependent for example on the layer thickness, on the wavelength of the photoinitiator that is used, and also on the wavelength of the radiation emitted by the UV radiation source.
Specifications DE 198 46 902 A1 and DE 101 63 545 A1 propose using EBC (electron beam) irradiation or UV irradiation from both sides in order to lower the resulting crosslinking profile and to provide virtually homogeneous crosslinking of thick UV-crosslinkable acrylate PSA layers in particular. However, the layers produced in this way still have a crosslinking profile, and, moreover, the process is very costly and inconvenient. Moreover, it would be virtually impossible to use in order to produce viscoelastic acrylate carriers; instead, the preparation of PSA layers in particular is described.
A disadvantage of viscoelastic acrylate carriers which exhibit a profile of crosslinking through the layer is their inadequate capacity for distributing stresses in a uniform way. One side is always either overcrosslinked or undercrosslinked. An exact balance can never be struck between adhesive and cohesive properties for the entire layer, but instead only for a small section.
EBC-crosslinked layers as well always exhibit a profile of crosslinking in accordance with the layer thickness and the material. With EBC-crosslinked layers as well it is impossible to set the crosslinking exactly. Nevertheless, EBC crosslinking proceeds without added photoinitiators, thereby removing some, although not all, of the disadvantages associated with the UV-irradiated layers. Depending on the accelerator voltage and on the thickness of the material to be irradiated, it is possible to vary the thickness of the irradiated layer. Layers above about 500 μm in thickness, particularly if filled with inorganic fillers such as glass balls, for example, can no longer be economically irradiated. Here, therefore, there is an upper limit on the layer thicknesses that are achievable.